1. Statement of the Technical Field
The invention concerns dielectric substrates for RF circuits, and more particularly dielectric substrates with selectively tailored electrical properties.
2. Description of the Related Art
RF circuits, including antennas, are commonly implemented on dielectric substrates. Materials commonly used for this purpose include commercially available low and high temperature cofired ceramics (LTCC, HTCC). For example, low temperature 951 cofire Green Tape(trademark) from Dupont(copyright) is Au and Ag compatible, has a acceptable mechanical properties with regard to thermal coefficient of expansion (TCE), and relative strength. It is available in thicknesses ranging from 114 xcexcm to 254 xcexcm and is designed for use as an insulating layer in hybrid circuits, multichip modules, single chip packages, and ceramic printed wire boards, including RF circuit boards. Similar products are available from other manufacturers.
LTCC substrate systems commonly combine many thin layers of ceramic and conductors. The individual layers are typically formed from a ceramic/glass frit that can be held together with a binder and formed into a sheet. The sheet is usually delivered in a roll in an unfired or xe2x80x9cgreenxe2x80x9d state. Hence, the common reference to such material as xe2x80x9cgreen tapexe2x80x9d. Conductors can be screened onto the layers of tape to form RF circuit elements, antenna elements and transmission lines. Two or more layers of the same type of tape is then fired in an oven. The firing process shrinks all of the dimensions of the raw part. Accordingly, it is highly important that the material layers all shrink in a precise, well defined way that will provide consistent results from one module to the next.
Due to the maturing of the antenna design process, the continued improvement of new antennas is most limited by the choice of substrate materials. However, development of new materials has proved difficult for a variety of reasons. One reason concerns certain incompatibilities of the physical properties associated with diverse materials that would otherwise be desirable to combine in a single dielectric composition. Often, the thermal coefficient of expansion (TCE), chemical properties of the materials, or sintering properties of the material may be inconsistent with one another. For example, different types of unfired ceramics such as Green Tape(trademark) will not fire well together because of different chemical and physical properties of the various different types of materials.
Still, new materials are needed for a variety of reasons. One such reason relates to the limited variety of specific electrical properties that are offered in commercially available dielectric substrate materials. Designers wishing to implement antennas or other RF circuit designs often find themselves constrained by the limitations of the material. For example, it may be desirable in a particular instance to implement an antenna array on a portion of a substrate having a particular value of permittivity, permeability or loss tangent. The requirements for these electrical properties can relate to form factor, electrical performance or other design issues. In any case, the limited choices of substrate materials that are presently available can require design compromises that are preferably avoided.
The invention concerns a method for fabricating a ceramic dielectric substrate for an RF circuit. The method can include selecting a substrate pattern and a plurality of ceramic materials to produce a textured substrate having one or more values of desired electrical properties such as permittivity, permeability, or loss tangent. The pattern will consist of a mosaic-like arrangement of dielectric pieces that are generally much smaller than a wavelength at the frequency of interest.
Advantageously, the substrate pattern can be selected for producing a dielectric substrate having one or more effective electrical properties that is different from a bulk electrical property of each individual one of the plurality of ceramic materials. Once the pattern and materials are chosen, the process can include firing the plurality of ceramic materials and cutting each of the plurality of ceramic materials into a selected size and shape to form dielectric pieces consistent with the desired pattern. An adhesive layer can be disposed on a base plate and the dielectric pieces can be arranged on the base plate in accordance with the predetermined pattern. After all the pieces have been placed, the adhesive can be cured to secure the dielectric pieces in place.
Subsequently, the method can include the step of polishing a surface of the substrate to obtain a selected textured substrate thickness. One or more conductive traces can thereafter be disposed on the substrate to define one or more RF circuit elements.
According to another aspect, the invention can include selecting two or more types of ceramic materials, each having a distinct set of electrical properties different from the remainder types of ceramic materials. A substrate pattern can also be selected. The substrate pattern can comprise at least two types of distinct substrate areas having the distinct sets of electrical properties of the ceramic materials. Each distinct area can be selected so as to have dimensions much smaller than a wavelength at a frequency of interest. The ceramic materials can thereafter be fired and then cut into a size and shape consistent with the distinct areas to form dielectric pieces. The process continues by selectively arranging the dielectric pieces on a base plate in accordance with the pattern to form the textured ceramic dielectric substrate. The completed textured substrate will have at least one effective electrical property at a frequency of interest that is different from each of said distinct sets of electrical properties of the ceramic materials.
According to yet another aspect of the invention, the a plurality of layers of the dielectric pieces can be arranged on the base plate. For example the method can include selectively arranging a second layer of the dielectric pieces on the first layer of textured substrate in a second pattern to produce a multiple layer textured substrate. The second pattern can be the same as the first patter, different from the first pattern or the same pattern but physically offset. A dielectric adhesive can be applied between adjacent layers of dielectric pieces to secure the layers together.